It is necessary to produce periodically an essentially disinfected condition in soft contact lenses so that bacterial organisms or their by-products will not cause harm to the wearer's eyes. Since the soft lens material is permeable to liquids, soaking the lens in strong germicidal solutions will result in the lens becoming impregnated with the solution, and this can lead to irritation to the user's eyes when the lens is worn. In general, it has been found difficult, if not impossible, to disinfect soft lens by treatment with chemical or biochemical solutions which will not cause eye irritation to at least some percentage of the wearers.
As an alternate means for producing the desired disinfected condition in the soft lens, heat has been used in the past. Since the lens must be kept immersed in physiologically normal saline solutions, or its equivalent, when it is not being worn, to prevent the lens material from drying out, heat is generally applied by first placing the lens in a saline solution in a container, and then placing the container in a heating unit.
The heating unit must raise the temperature of the saline solution and immersed lens in the container to the required temperature, hold the lens at or above this temperature for the required time, and then allow the lens to cool to ambient temperature. Typical values of the time and temperature deemed suitable for producing the disinfected condition require that the lens to be maintained at or about 80.degree. C., for a period of 10 minutes or more. Since aging of the lens material is accelerated by excessive temperatures and/or by extended time at elevated temperatures, it is desirable that the heating unit be controlled so that excessive temperatures, or excessive time at elevated temperatures, will not shorten the life of the lens.
It is also desirable that the lens user be able to check periodically on the proper operation of the heating unit to insure that the unit has reached the desired temperature. An indicator light on a typical prior art electrical heating unit only tells the user that the unit has been turned on. The light may well function normally even if the heating element in the unit is defective and the lens has not reached the proper disinfecting temperature.
The Bowen U.S. Pat. No. 4,178,499 referred to above provides a simple and inexpensive indicator assembly for use in a heating unit of the type disclosed in Seitz U.S. Pat. No. 4,158,126, by which the user may observe when the proper temperature has been reached after the unit has been energized. The indicator disclosed in the Bowen patent comprises an indicator strip which is mounted within the interior of the heating unit, and which is displaced from a window in the wall of the unit. The strip is not visible through the window in the unit so long as the wax within the unit is in a solid opaque state. However, when the wax is heated to its molten state, it becomes transparent, and the indicator is then visible. The present invention provides a simple and inexpensive indicator which operates on the same principle as the indicator of the Bowen patent, but which is housed in a separate plug so as to be easier and less expensive to produce.